In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict control procedures in the manufacturing processes. The use of composite materials in military fighter aircraft has increased. For instance, in the F14, 0.8% of the total structural weight of the aircraft was made from composite. In the F15, F16 and F18, 1.6%, 2.5% and 9.5%, respectively, was composite. In the Joint Strike Fighter (JSF), composite materials comprise more than 50% of the aircraft structural weight. Similarly, large transport aircraft use of composites has increased as well. In 1985, 3% of the aircraft's structural weight was composite and this percentage has only increased.
Composite structures are typically held together by adhesives, but this has not always been found practical. In conventional aluminum aircraft, approximately 25 fasteners per square foot of external surface area are used. In one estimate, composite aircraft use approximately 12 fasteners per square foot of outside surface area.
Traditional fasteners require internal threads within the substructure. The use of threaded fasteners within composite material structures requires material changes to accept the fastener. Otherwise, component life of the composite is significantly reduced at the thread roots of the threaded fastener coupled to non-reinforced composite material. These internal threads require that the weight be increased due to material changes of the internal structure to a stronger material as well as increasing the size of the structure to limit crack initiation at the threaded roots of the mechanical fastener. Additionally, when composite materials are used, this results in a limited repair capability and reduced component life.
Traditional pylon attachment design have included a threaded structural insert serving as a repairable wear surface installed in the substructure and secured with securing adhesives. External load attachment creates areas of high structural stress and loading. For example, in an aircraft, pylon attachments experience high structural loading and stress, and may be located within areas of an aircraft that already experience high structural loading such as a wing-fold rib. The wingfold rib experiences high loading during flight, takeoff and landing.
Specially-shaped and modified metal fasteners have been used in composite structures previously. These fasteners result in increased weight by replacing portions of the composite structure. Additionally, the composite structure must be modified to accept these modified fasteners. These fasteners may be unnecessarily strong, heavy, expensive and undesirably reflective to radar detection devices. Thus, a need exists for improved fasteners for use with composite structures having the necessary strength characteristics.